The power beyond type system is a series of valves. Fluid exits the pump, goes into the first valve's inlet port.(backhoe on bx 24) if the spool is centered(no lever moved) hydraulic flow will exit the power beyond port. From there it goes into the next (loader control) valve's inlet port. Again if the spools stay centered, flow will exit the valve's power beyond port and goes to the transmission case and into the spool for the 3 pt hitch, again if the spool is centered the fluid will pass through the valve and be returned to the sump.
Installing a open center valve into the system will cause no pressure at any valve downstream from the open centered valve because the open center is open to atmospheric presssure via the return line and all pump flow is returned to the sump.
The power beyond spool is not a plug. It seals off the return line from the pressure line in a open center valve. It still allows through flow for other valves. If properly connected it will NOT cause the pressure relief valve to open. Closed center valves allow NO flow unless a spool is actuated, this WILL cause the relief to run constant, this is very bad, it WILL damage a BX.
Another factor is most spool valves have a integral relief valve of their own. This relief pressure setting must exceed the tractors relief pressure or all down srtream valves will relief at the lower pressure.
Then there is the return line. All this does is return fluid from the cylinders to the sump.
To add a power beyond valve to a BX24 you will need a control valve set up for power beyond.
Connect the valve's inlet port to the short hose that stays on the tractor.
This is the pump outlet.
Connect the power beyond port to the fitting you would connect the short hose to when not using the backhoe. This is the pressure line to the rest of the system.
Connect the return line to the remaining fitting on the tractor. This is direct to the sump.
The valve is now in series with the power beyond system and will allow proper function of the system.
I hope this helps, Dave